Beggars Bazooka version but for speed, the longer you hold, the faster it will go. If you just click it falls on your feet and damages you. If you hold too much it explodes and launches you back.

I Thought You Said The Longer You hold The Faster, But It Explodes If You Hold It Long? Logic.

Read it again, and please don't capitalize everything otherwise it becomes hard to read.
I said that the longer you hold the faster it will go, but if you hold TOO MUCH, it explodes. Just like the Loose Cannon.
Logic.

What if you can charge it and if it is charging him long enough he fires a potato that stuns the enemy

How Long Is Too Much?

The Loose Cannon's treshold would be a nice example, but in the end it would be up to VALVe's TF2 team to decide.

I feel like stats should be;
Rockets go at a slight arch
10- reload
30+ movement speed
Shoots exploding potatos

While the arc shooting shouldn't be there (that's demoman's job, unless it's an horizontal shooting like a boomerang, would be interesting), I fully approve of potato

Homing rockets
Full damage at maximum range
-30% damage
-50% clip size

Sort of like a sniper type gameplay with the soldier.

Custom projectile model of potato stuck through with nails (really crazy with spikes coming out of it all over, even though it doesn't make sense mechanically). The "explosion" is a custom effect of the potato just blowing apart on impact and scattering its nails around at high speed (no flame).

+25% move speed
-70% damage
+40% projectile speed
-40% blast radius
Does not use ammunition, just a single shot that infinitely reloads at high speed, for an effective sustained ROF ~1.5x stock.
Direct hit causes bleed for 5 seconds on enemy targets (in addition to explosion) and at range gives a distance-scaling minor health restore on teammates (20-65, with a good deal of distance before it gives anything).
Critical hits cause bleed on all targets in blast radius.
Taunt to eat a nail-filled potato, restoring full health but marking you for death for 10 seconds, during which time you cannot use the weapon.
The gun only propels you a bit when jumping (eg Detonator or Scorch Shot).

I think that it should be this: the longer your hold the fire key, the longer the rocket or potato will fly.

First shot does no damage but marks the victim for death
-50% clip size (2 rockets for those of you that cant do math)
+25% reload speed

With Sergio's idea, but it'd be cool if it had an arch that wasn't too noticable. However after a certain charge time it'd be like a normal rocket.

I think that it should be a nothlethal launcher (-100% damage btw) but it works like the sandman in the effect that it fires misc items that stun opponents. Like, yes, a potato launcher

It could be like the beggars bazzooka but because it's a "D.I.Y" weapon, it could be like 4th of July/chinese new year celebrations and have firecrackers. These firecrackers would replace the rockets but there would be a lot more of them firing off in different directions. They would do a lot less damage but the longer the charge is held, the more accurate it will be. However, if you hold it down long enough, it would explode back in your face dealing explosive damage.